import { getRoundingMethod } from "../_lib/getRoundingMethod/index";
import { differenceInDays } from "../differenceInDays/index";
import type { ContextOptions, DateArg, RoundingOptions } from "../types";

/**
 * The {@link differenceInWeeks} function options.
 */
export interface DifferenceInWeeksOptions
  extends RoundingOptions,
    ContextOptions<Date> {}

/**
 * @name differenceInWeeks
 * @category Week Helpers
 * @summary Get the number of full weeks between the given dates.
 *
 * @description
 * Get the number of full weeks between two dates. Fractional weeks are
 * truncated towards zero by default.
 *
 * One "full week" is the distance between a local time in one day to the same
 * local time 7 days earlier or later. A full week can sometimes be less than
 * or more than 7*24 hours if a daylight savings change happens between two dates.
 *
 * To ignore DST and only measure exact 7*24-hour periods, use this instead:
 * `Math.trunc(differenceInHours(dateLeft, dateRight)/(7*24))|0`.
 *
 * @param laterDate - The later date
 * @param earlierDate - The earlier date
 * @param options - An object with options
 *
 * @returns The number of full weeks
 *
 * @example
 * // How many full weeks are between 5 July 2014 and 20 July 2014?
 * const result = differenceInWeeks(new Date(2014, 6, 20), new Date(2014, 6, 5))
 * //=> 2
 *
 * @example
 * // How many full weeks are between
 * // 1 March 2020 0:00 and 6 June 2020 0:00 ?
 * // Note: because local time is used, the
 * // result will always be 8 weeks (54 days),
 * // even if DST starts and the period has
 * // only 54*24-1 hours.
 * const result = differenceInWeeks(
 *   new Date(2020, 5, 1),
 *   new Date(2020, 2, 6)
 * )
 * //=> 8
 */
export function differenceInWeeks(
  laterDate: DateArg<Date> & {},
  earlierDate: DateArg<Date> & {},
  options?: DifferenceInWeeksOptions | undefined,
): number {
  const diff = differenceInDays(laterDate, earlierDate, options) / 7;
  return getRoundingMethod(options?.roundingMethod)(diff);
}
